1. Field of the Invention
This invention relates to a tone signal generating system for use in communication apparatus, and more particularly a tone signal generation system wherein a tone signal generating capability is provided to memory means provided in the system.
2. Description of the Prior Art
A tone squelch system in which a tone signal generating circuit is provided in a transmitter and in which a squelch circuit contained in a receiver is controlled in accordance with a tone signal generated by the tone signal generating circuit, including a tone control system that controls the transmission apparatus have been proposed.
FIG. 1 is a block diagram showing such a transmitter/receiver, particularly a multichannel access system utilizing the tone squelch system described above.
During the waiting state, a frequency synthesizer of the transmitter/receiver, not shown, is controlled so as to acquire a predetermined control channel. When there is a call signal to each transmitter/receiver or to a group in which a plurality of transmitter/receivers are involved, the synthesizer is caused to acquire a speaking channel which is indicated by a signal from the control channel. Then, the transmitter receiver transmits or receives the signal through the speaking channel.
Transmission and reception operations after a speaking channel has been selected will be described as follows, referring to the accompanying drawings.
A transmitter 1 consists of a transmitting circuit 5 which modulates a voice signal produced by a microphone 2 and transmits the modulated signal through an antenna 4 via an antenna transmit/receive switch 3, a tone signal generating circuit 8 comprising an oscillation circuit 6 and a phase-reversal circuit 7, a press to talk signal delay circuit 9, and an inverter 11 connected between the press talk signal delay circuit 9 and a press to talk switch 10. One end of the press to talk switch 10 is grounded, while the other end is supplied with a predetermined positive voltage via a resistor 12. The press to talk signal delay circuit 9 consists of an inverter 13, an AND gate circuit 14, a delay circuit 15 and OR gate circuit 16.
After a speaking channel has been selected, when the press to talk switch 10 is closed at time t.sub.1 the output of the inverter 11 becomes a high level. This output signal is applied to one input of the OR gate circuit 16 as a transmission enable signal A shown in FIG. 2a. The output B (see FIG. 2b) of the OR gate circuit 16 becomes H (high) level, and is applied to the oscillation circuit 6 so as to produce a tone signal which is supplied to the phase-reversal circuit 7. At this time the output of the inverter 13 is at a low level, and the output of the AND gate circuit 14 also becomes the low level. Consequently, the phase of the tone signal is not reversed and the tone signal is input, as it is, to the transmitting circuit 5 as a tone signal D shown in FIG. 2d. When the press to talk switch 10 is opened at time t.sub.2, the output of the inverter 11, i.e., transmission enable signal A, becomes low, whereas the output of the inverter 13 becomes high. The output of the OR gate circuit 16 is maintained at the high level by the delay circuit 15 during an interval .DELTA.T between times t.sub.2 and t.sub.3. Accordingly, the output of the tone signal D from the oscillating circuit 6 will be continued up to time t.sub.3, and the output of the OR gate circuit 16 is also supplied to the transmitting circuit 5 as the delay request signal B shown in FIG. 2b. During an interval .DELTA.T between times t.sub.2 and t.sub.3, the output C (See FIG. 2c) of the AND gate circuit 14 becomes high level so as to send a phase-reversal control signal to the phase-reversal circuit 7, whereby the phase-reversal circuit 7 reverses and outputs tone signal D. The transmitting circuit 5 transmits a transmission signal containing a non-reversed and reversed tone signal D through the antenna. In the waiting state when no control channel is selected, the transmitter 1 is controlled to not send the tone signal irrespective of the ON-OFF states of the press to talk switch 10.
Receiver 21 shown in FIG. 1 comprises a receiving circuit 22 which demodulates a signal received through the antenna 4 and antenna transmit/receive switch 3, a low frequency amplifier 24 supplying the demodulated signal to a loud speaker 23, an analogue switch 25 interposed between the receiving circuit 22 and the low frequency amplifier 24, a first squelch signal generating circuit 26 generating a first squelch control signal which may be used to control the analogue switch 25, a second squelch control signal generating circuit 32 generating a second squelch control signal which may be used to control the analogue switch 25, and a selection switch 33 which selects either one of the first and second squelch control signal generating circuits 26 and 32. The first squelch control signal generating circuit 26 generates a first squelch control signal in response to a tone signal and is composed of a bandpass filter 27, a tone signal detector 28, a phase reversal detector 29, an inverter 30 and an AND gate circuit 31. The second squelch control signal generating circuit 32 comprises a noise squelch circuit which produces a squelch signal based upon a noise signal.
A control circuit 40 is provided for controlling the operation of the transmitting circuit 5, the receiving circuit 22 and the selection switch 33, etc. and functions to control selection operations of the speaking channel and a control channel, transmitting and receiving operations of various data signals transmitted through the channel, processing operations based on the data signals and control of the operation of the selection switch 33, for example.
After selecting a speaking channel when a signal E, shown in FIG. 3a, containing a tone signal is received by the receiving circuit 22, a signal F (see FIG. 3b) is output by the receiving circuit 22 is supplied to the low frequency bandpass filter 27 and the phase reversal detector 29. When the tone signal is positive, the inverter 30 outputs a high level signal so that the output of the AND gate circuit 31 becomes the high level, thus supplying to the analogue switch 25 a squelch control signal G as shown in FIG. 3c. Consequently, the analogue switch 25 is turned ON at time t.sub.4 so that the received signal is supplied to loudspeaker 23 via low frequency amplifier 24. Thereafter when the phase of the tone signal F is reversed at time t.sub.5, the phase reversal is detected by phase reversal detector 29 and the output of the inverter 30 becomes a low level. As a consequence, the output of the AND gate circuit 31, that is the squelch control signal G, becomes the low level. This signal G is applied to the analogue switch 25 via selection switch 33 to turn OFF the analogue switch, whereby the demodulated output signal H shown in FIG. 3d becomes the low level. The reason that the phase of the tone signal is reversed at time t.sub.2 (see FIG. 2) at which the press to talk switch 10 is turned OFF lies in that no detection delay occurs when compared with a case in which absence of the tone signal is detected at time t.sub.2. The control circuit 40 controls the selection switch 33 such that it selects the first squelch control signal generating circuit 26 when a speaking channel is selected, whereas when a control channel is acquired, it selects the second squelch control signal generating circuit 32.
FIG. 4 illustrates one example of a prior art tone signal generating circuit 8 provided for the transmitter 1 shown in FIG. 1, in which an oscillator 6 is composed of a sine wave oscillator 35, a variable resistor 36, a capacitor 37 and an analogue switch 38. Thus the oscillator 6 produces a tone signal S1 (see FIG. 5a) whose frequency is determined by the values of the variable resistor 36 and capacitor 37. As shown in FIG. 5b, the analogue switch 38 sends a tone signal S2 to a phase reversal circuit 7 only when the output of the OR gate circuit 16 of the press to talk signal delay circuit 9 is at the high level.
The phase reversal circuit 7 is composed of analogue switches 39 and 40, inverter 41 and operational amplifier 42 having a unity gain. When the output of the AND gate circuit 14 of the press to talk signal delay circuit 9 is at a low level, the switch 40 is ON while the switch 39 is OFF, whereby a tone signal S3 shown by FIG. 5c is applied to the noninverting terminal of the operational amplifier 42. When the output of the AND gate circuit 14 becomes the high level, the switch 39 is ON and the switch 40 is OFF so that a tone signal S4 shown by FIG. 5d is applied to the inverting input terminal of the operational amplifier 42. Accordingly, a tone signal S5 whose phase has been inverted at time t.sub.2 as shown in FIG. 5e appears at the output terminal 43 of the tone signal generating circuit 8 and the tone signal S5 is sent out to the transmission circuit 5.
In the tone signal generating circuit 8 shown in FIG. 4, because its oscillator 6 contains sine wave oscillator 35 which is of a complicated construction, and moreover because its oscillation frequency is determined by the values of variable resistor 36 and capacitor 37, these component parts should be accurate and should have excellent temperature characteristics, causing the components to be expensive. If transmission apparatus equipped with a tone signal generating circuit 8 is mounted in a motor car, due to the large temperature variations in the motor car, the accuracy degrades due to the temperature characteristics. To prevent such degradation it is necessary to use expensive parts having better temperature characteristics. Moreover, as the accuracy of the tone signal degrades it is impossible to transmit a large number of tone signals in a limited tone signal frequency bandwidth, thus failing to efficiently use the frequency bandwidth. Moreover, in order to reverse the tone signal it is necessary to provide an exclusive phase inverting circuit 7. In addition, it is necessary to use three analogue switches 38, 39 and 40 for the purpose of controlling the tone signals, which creates problems regarding space and cost.